Allison A. Wing

As part of BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment), the German research vessel FS Meteor navigated the moist tropics of the Atlantic Ocean for 40 d in summer 2024, with an east-west trajectory. The journey started in the port of Mindelo, Cape Verde, on 16 August, and finished in the port of Bridgetown, Barbados, on 24 September. The objective was to measure properties of the atmosphere, upper ocean, and air-sea interface within the Intertropical Convergence Zone (ITCZ), under a variety of wind, convection, and sea surface temperature regimes. Using a set of 29 instruments/platforms, BOWTIE sampled the near-surface conditions of the atmosphere and ocean with high temporal resolution. This included continuous measurements of: the 2D wind field within the lowest 2 km of the atmosphere, near-surface ocean currents, cloud and precipitation properties. Profiles of the ocean state and atmospheric thermodynamics and kinematics were obtained both continuously and at discrete intervals. Furthermore, biochemical properties of the upper ocean were sampled during dedicated stations. Complementing BOWTIE observations, FS Meteor hosted further dedicated field campaigns for 3D cloud and precipitation properties, as well as intensive measurements of the atmospheric boundary layer. This manuscript provides an overview of the extensive instrumentation and data collected during BOWTIE. In addition, it addresses two key aspects based on these observations. First, it examines the range and uncertainties of selected quantities measured by multiple instruments, including column-integrated water vapor, rain detection, surface ocean currents, and sea surface temperature. Second, it illustrates the diversity of sampled weather regimes through two representative cases: calm doldrum conditions and a gusty, precipitating convective state. The links and DOIs of the data sets presented in this study are listed in the code and data availability section.

Clouds over the tropical Atlantic remain under-represented in ground-based observational studies, despite their key role in the regional radiative and hydrological budgets. This observational gap contributes to substantial uncertainties in the representation of these clouds in climate models. The recent BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment) campaign aimed to address this deficiency through a six-week, ship-borne measurement campaign across the tropical Atlantic, spanning from Cape Verde to Barbados. This study investigates the influence of Saharan Air Layers (SALs) on cloud properties using vertically pointing remote sensing observations collected during the research cruise. Cloud types are classified and combined with Cloudnet-derived microphysical products to assess both macrophysical and microphysical cloud characteristics. The analysis reveals a shift in the cloud characteristics toward less convective regimes during SAL presence, accompanied by changes in cloud base height (CBH) and cloud top height (CTH). Microphysically, the suppression of convection associated with SAL leads to a greater vertical separation of cloud layers, meaning that droplet and ice effective radii (DER and IER) differ across layers. However, hydrometeor populations across all observed layers are comparable to those observed in marine cloud regimes reported in previous studies. Overall, the thermodynamic structure of the SAL emerges as the dominant driver of the observed changes in cloud properties. These findings highlight the importance of accounting for SAL influences in understanding cloud development over the tropical Atlantic and improving their representation in atmospheric models.

Clouds over the tropical Atlantic remain under-represented in ground-based observational studies, despite their key role in the regional radiative and hydrological budgets. This observational gap contributes to substantial uncertainties in the representation of these clouds in climate models. The recent BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment) campaign aimed to address this deficiency through a six-week, ship-borne measurement campaign across the tropical Atlantic, spanning from Cape Verde to Barbados. This study investigates the influence of Saharan Air Layers (SALs) on cloud properties using vertically pointing remote sensing observations collected during the research cruise. Cloud types are classified and combined with Cloudnet-derived microphysical products to assess both macrophysical and microphysical cloud characteristics. The analysis reveals a shift in the cloud characteristics toward less convective regimes during SAL presence, accompanied by changes in cloud base height (CBH) and cloud top height (CTH). Microphysically, the suppression of convection associated with SAL leads to a greater vertical separation of cloud layers, meaning that droplet and ice effective radii (DER and IER) differ across layers. However, hydrometeor populations across all observed layers are comparable to those observed in marine cloud regimes reported in previous studies. Overall, the thermodynamic structure of the SAL emerges as the dominant driver of the observed changes in cloud properties. These findings highlight the importance of accounting for SAL influences in understanding cloud development over the tropical Atlantic and improving their representation in atmospheric models.

Cloud processes constitute one of the key uncertainties for climate change projections. The fourth iteration of the Cloud Feedback Model Intercomparison Project, CFMIP4, contributes to the Coupled Model Intercomparison Project phase 7 (CMIP7), by providing a set of global climate model experiments aiming to enhance our understanding of clouds, circulation and climate sensitivity, thereby informing improved projections of future climate change. CFMIP4 targets four knowledge gaps: (1) Physical mechanisms of cloud feedback and adjustment; (2) Dependence of cloud feedback and adjustment on climate base state and on the nature of the forcing; (3) Coupled mechanisms of the sea-surface temperature “pattern effect”; and (4) Coupling of clouds with circulation and precipitation. CFMIP4 contributes four CMIP7 Assessment Fast Track experiments that are central to the quantification of climate feedback and sensitivity in past, present and future climates, essential for process understanding and model evaluation. Furthermore, CFMIP4 supports the joint analysis of models and observations through a data request that includes process and satellite simulator output.

Here we investigate elevated stable layers that frequently appeared at an altitude near 6 km in soundings obtained during the ORCESTRA field campaign. This observation deviates from the existing literature, which emphasizes that tropical mid-level stable layers are collocated with the 0°C level at ~4-5 km. Our analysis demonstrates a strong relationship between mid-level stability and the change in relative humidity across the stable layer (decreasing upwards). Mechanism-denial tests with an infrared radiative transfer model support the hypothesis that vertical changes in relative humidity are sufficient to drive the formation of mid-level stable layers through differential radiative heating. We compare the frequency and strength of these stable layers to those in other ocean basins utilizing data from other field campaigns. These comparisons highlight the correlation between vertically decreasing relative humidity and enhanced stable layers, as well as the frequency of stable layers above the 0°C level across ocean basins.

Cloud-radiative feedback plays a critical role in climate and the formation of extreme-weather-producing systems such as tropical cyclones (TCs; e.g., hurricanes). Yet, the convective-scale processes by which this feedback promotes the upscale growth of tropical convection, including TC formation, remain poorly understood. Here, we leverage multiple numerical modeling frameworks to show that longwave warming by stratiform and anvil clouds promotes upscale development by suppressing downdrafts. As byproducts of deep convection, these cloud systems are responsible for the majority of evaporatively-cooled downdrafts in tropical mesoscale convective systems (MCSs), which help terminate convection by stabilizing the troposphere. We show, however, that their radiative warming is balanced by ascending motion that moistens the column and suppresses these downdrafts, therefore promoting convective maintenance and upscale growth by increasing precipitation efficiency (ε). We demonstrate this feedback through cloud-radiative feedback mechanism-denial tests in three distinct convection-permitting model frameworks, including TC case studies, an aquaplanet, and non-rotating radiative-convective equilibrium (RCE). When clouds and longwave radiation interact, moist convection exhibits greater positive trends in area coverage and mean convective-system size. This feedback promotes more frequent TC development across intensity thresholds in the aquaplanet framework. These findings underscore the active role of stratiform clouds in tropical convective evolution through the link between their longwave radiative warming and ε. They help explain why expanding stratiform raining area in nascent TCs can signal imminent genesis and intensification, and why model predictions of radiatively-driven tropical systems such as the Madden–Julian Oscillation strongly hinge on the representation of a robust stratiform regime.

Clouds over the tropical Atlantic remain under-represented in ground-based observational studies, despite their key role in the regional radiative and hydrological budgets. This observational gap contributes to substantial uncertainties in the representation of these clouds in climate models. The recent BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment) campaign aimed to address this deficiency through a six-week, ship-borne measurement campaign across the tropical Atlantic, spanning from Cape Verde to Barbados. This study investigates the influence of Saharan Air Layers (SALs) on cloud properties using vertically pointing remote sensing observations collected during the research cruise. Cloud types are classified and combined with Cloudnet-derived microphysical products to assess both macrophysical and microphysical cloud characteristics. The analysis reveals a shift in the cloud characteristics toward less convective regimes during SAL presence, accompanied by changes in cloud base height (CBH) and cloud top height (CTH). Microphysically, the suppression of convection associated with SAL leads to a greater vertical separation of cloud layers, meaning that droplet and ice effective radii (DER and IER) differ across layers. However, hydrometeor populations across all observed layers are comparable to those observed in marine cloud regimes reported in previous studies. Overall, the thermodynamic structure of the SAL emerges as the dominant driver of the observed changes in cloud properties. These findings highlight the importance of accounting for SAL influences in understanding cloud development over the tropical Atlantic and improving their representation in atmospheric models.

The Radiative‐Convective Equilibrium Model Intercomparison Project (RCEMIP) phase I provided insights into the tropical radiative‐convective equilibrium state across an unprecedented ensemble of models that includes both those with parameterized and explicit convection. RCEMIP phase II (RCEMIP‐II) introduces mock‐Walker simulations to examine how sea surface temperature (SST) gradients and the forced circulation they theoretically induce interact with the structure of convection and mean climate state. This overview paper presents descriptions of and initial results from 17 numerical models regarding clouds, convective organization, and circulation structure. Results regarding warming the mean SST in the presence of an SST gradient generally mirror those from RCEMIP‐I but a larger range of dynamical circulations, along with additional phenomena, are possible in RCEMIP‐II. Increases in the SST gradient while the mean SST is held constant lead to more convective organization across many models, as measured by a number of organization metrics. Increases in the SST gradient also tend to decrease the frequency of high, thin clouds while increasing the frequency of low clouds, including the possibility of stratocumulus, which were not seen in RCEMIP‐I. RCEMIP‐II also provides an estimate of the influence of a pattern effect, suggesting increased negative climate feedbacks if increases in the SST gradient occur alongside mean warming. However, the SST gradient's inability to fully constrain the circulation structure allows simulations to show varying importance of self‐aggregation and forced clustering over the warm pool, resulting in extreme variety in the circulation structures and important climate variables in the organized state across the model ensemble.

As part of BOWTIE (German: Beobachtung von Ozean und Wolken - Das Trans ITCZ Experiment), the German research vessel FS Meteor navigated the moist tropics of the Atlantic Ocean for 40 days in summer 2024, with an east-west trajectory. The journey started in the port of Mindelo, Cape Verde, on August 16, and finished in the port of Bridgetown, Barbados, on September 24. The objective was to measure properties of the atmosphere, upper-ocean, and air-sea interface within the Intertropical Convergence Zone (ITCZ), under a variety of wind, convection, and sea surface temperature regimes. Using a set of 29 instruments/platforms, BOWTIE sampled the near-surface conditions of the atmosphere and ocean with high temporal resolution. This included continuous measurements of: the 2-D wind field within the lowest 2 km of the atmosphere, near-surface ocean currents, cloud and precipitation properties. Profiles of the ocean state and atmospheric thermodynamics and kinematics were obtained both continuously and at discrete intervals. Furthermore, dedicated stations sampled biochemical properties of the upper-ocean. Complementing BOWTIE observations, FS Meteor hosted further dedicated field campaigns for 3D cloud and precipitation properties, as well as intensive measurements of the atmospheric boundary layer. This manuscript provides an overview of the extensive instrumentation and data collected during BOWTIE. In addition, it addresses two key aspects based on these observations. First, it examines the range and uncertainties of selected quantities measured by multiple instruments, including column-integrated water vapor, rain detection, surface ocean currents, and sea surface temperature. Second, it illustrates the diversity of sampled weather regimes through two representative cases: calm doldrum conditions and a gusty, precipitating convective state.